Cliff Mass Weather and Climate Blog

Saturday, March 17, 2018

Some call it the "March Miracle"--the large amounts of precipitation and snow that fallen over California the past month. The Sierra Nevada got hit by 2-5 feet this week alone. And this is a miracle that is not over, with a strong atmospheric river poised to occur over the next week.

Here are snow water equivalent maps (amount of water in the snowpack) from the National Snow Analysis for February 17 and March 17. A huge increase in both depth and coverage.

The origin of the wet bounty over California was a shift in the large scale atmospheric circulation, with high pressure moving further offshore and persistent troughing (low pressure) developing over the West Coast. To illustrate, here is mean upper level (500 hPa, about 18,000 ft) heights for March 10-16th. The left shows the heights (analogous to pressure) and the right shows the deviation (or anomaly) from normal. A trough (low heights) is found right off the West Coast...that is culprit.

Well, this pattern is not going away...and in fact, it will amplify in a few days and our friends in California need to get prepared. A trough will form west of California with strong southwesterly flow on its south side, which will entrain large amounts of tropical moisture (see map for 11 PM Tuesday below)

As a result, a strong atmospheric river will develop, which will bring tropical moisture into central and southern California (see plot of total moisture in the vertical for 5 PM Tuesday)

How much precipitation will those folks in California enjoy? Here is the forecast accumulated precipitation over California for the next 7 days from the European Center Model: up to 6-8 inches in the Sierra Nevada and coastal mountains.

And plenty around southern CA, such as the mountains that surround LA.

Snowfall? Up to 3- 4 feet in the Sierra Nevada (see below)

With all the recent snow, the Sierra Nevada snowpack is about 60% of normal. It will get much closer to normal after the next few weeks. And the reservoirs, already around 100% of normal, will get topped off for the upcoming dry summer.

The only negative of all this precipitation is that it will encourage the growth of grasses, which can enhance the potential for fire danger next fall after it has dried out.

Here in the Northwest, our situation will be far less exciting than for California, with much of the big action going south of us. The 7-day totals show the heaviest precipitation over western Oregon but only light precipitation over the Puget Sound area and the San Juans.

Thursday, March 15, 2018

Last week a paper (Mote et al., 2018) was published in the journal Climate and Atmospheric Science claiming "dramatic" declines in western U.S snowpack (you can access it here).

The article had all kinds of scary details. The loss in water resources would be "comparable in volume to the West’s largest man-made reservoir, Lake Mead" and the losses would be so great that "new reservoirs cannot be built fast enough to offset the loss of snow storage."

The principal author's (Phil Mote) institution put out a press release that amplified the message, with the lead author noting that:

“It is a bigger decline than we had expected,”

The media headlined this "dramatic" loss of western snowpack, with hundreds of stories in major outlets around the world, allowing millions of people to learn about the bad news. Here are a few examples. I could show you a hundred more, but you get the point.

But fortunately, the "dramatic" headlines and all the hype are not correct.

There has been no "dramatic" loss of western snowpack during the past century, but rather a relatively slow, steady decline. And as I will demonstrate, the scary paper's own research supports a less apocalyptic interpretation. As does other research in the peer-reviewed literature.

But let's get our terminology down straight. What does dramatic mean? Let's look at a typical definition (Oxford)

Since we are not talking about a thespian document, definition (2) is the one we want: an event or circumstance that is sudden and striking. Has the trend of snowpack over the western U.S. during the past decades been sudden and striking?The answer to this question is really important. Many politicians and activist organizations are claiming that we have experienced a rapid decline in western snowpack driven by global warming. And an accurate knowledge of snowpack changes is clearly important for making decisions about water resources. And what about the future of western U.S. snowpack?

The Mote et al. paper uses two approaches to evaluate past snowpack changes over the western U.S. The first is to examine snowpack changes based on direct measurements. The problem with that approach is that there are only a limited number of stations and the number and distribution of such stations have changed considerably over time.

The second makes use of a snow/hydrology simulation model called VIC (Variable Infiltration Capacity) model, developed by Professor Dennis Lettenmaier of UCLA (and formerly the UW). This model uses precipitation and temperature inputs (there is a LOT more of these than snowpack measurements) to simulate the changing snowpack.

Below is a figure from the Mote et al paper showing the snowpack (actually the snow-water-equivalent or SWE) over the western U.S. on April 1 each year from 1915 to 2014 using the VIC model approach. They also fitted a line to the variation over time. You will note that there is huge amount of variability year to year, with an apparent slow decline in snowpack over the past century. Specifically, they found a 21% decline over the past century or 2.1% decline per decade. Hardly seems dramatic. I should note that April 1 snowpack is a frequently used measure, since in the west snowpack generally peaks around then, and thus April 1 snowpack is a good measure of the water availability for the upcoming warm season.

Now imagine their line was not there. In fact, you don't have to imagine, I have done it for you! There doesn't seem to be any decline during the past few decades...if anything, the snowpack seems to be increasing.

In fact, here is the same figure, with only the last 40 years shown. No decline, dramatic or otherwise is apparent. Where did that headline come from?

Now a completely independent analysis of long-term snowpack trends over the Northwest U.S. is found in a peer-reviewed paper in the Journal of Climate (A New Look at Snowpack Trends in the Cascade Mountains by Stoelinga et al...found here). They used a statistical approach to secure the snowpack from temperature, precipitation, and streamflow instead of the physical model (VIC) mentioned above. But the same general idea. Their results for 1930 to 2007 are quite similar to those found in the Mote et al (2018) paper, with a 23% decline for the entire period, and increasing snowpack since 1975. I repeat, increasing.

Furthermore, they went one step further and tried to remove natural variability (like the Pacific Decadal Oscillation) and got the April 1 snowpack trend shown below. Plenty of variability and a very slow decline (16% over the period). About a 2% decline per decade...similar to the Mote et al. VIC results.

Nothing large, nothing sudden, nothing dramatic. 2% loss per decade. No acceleration of snowpack loss. And as I will explain late, this make perfect sense considering that the Pacific Ocean is just west of us.

But what about snowpack observations over the West?

As noted by Mote et al., there is a major problem using such observations: the number of measurement sites is small and their numbers and distributions have changes substantially over the past 50 years. To illustrate, here is a figure from the supplementary material from the Mote et al paper, showing changes in the number of observations for various subregions. Few observations before 1940 and a major increase in the 60s and 70s. The numbers have been relatively stable since roughly 1975-1980.

With those issues noted, below is a plot from Mote et al of the observed April 1 snowpack for three western regions: the Cascades, the Rockies, and California. The circles are the average snowpack for each region (ignore the red xs and red line...that is for the VIC model which we already covered). I removed a blue trend line from these figures--I want you to make your own appraisal of the trends. Specifically, look at the period since 1980, when the observational network as relatively stable.

It is clear that there is little April 1 snowpack trend in the observations for the last 35 years. Yes, 2015 has a very poor snowpack...but that was an isolated outlier....for climate studies we must look at the trend...and there simply has not been much of trend. Just a lot of variability.

As an independent check on the observed trend in April 1 snowpack, research meteorologist Mark Albright, past WA state climatologist, did his own analysis of the April 1 snowpack changes over Oregon, Washington, Idaho, and Montana using the USDA Snotel observing stations. He considered the period 1984-2017, since the SNOTEL network expanded into the early 1980s. A seen below, there is virtually no trend over that period (and I might note that 2018 looks like an above-normal year).

Now to beat a dead horse, here is one more observed record of mountain snowpack, one encompassing a very long period (1879-2017): the snowpack at Donner Summit, high in the Sierra Nevada (this is from the Central Sierra Nevada site associated with the University of California, Berkeley). The color bars are April 1 snowpack). More snowpack in the late 1800s, but only a slightly downtrend during the past several decades.

Now you might ask, why has western snowpack been so stable if the earth is warming? A very good question.

A major part of the answer is that the eastern Pacific has NOT warmed very much and our temperatures are substantially controlled by the eastern Pacific surface temperatures. To illustrate the lack of warming, here are the surface air temperatures from the NASA/GISS website, showing the trends from 1977-2015. The eastern Pacific actually cooled during that period.

This pattern is similar to that indicated in climate models driven by increasing CO2. The Arctic warms up more than anywhere, land warms more quickly than oceans, and eastern oceans generally warm the slowest.

I know that some of you are unhappy with the above analysis, even though the evidence is pretty compelling that the snowpack has not been dramatically reduced the past few decades. You have heard the constant drumbeat from the media, some activist groups, and a few scientists who should know better.

But before some start calling me names (e.g., climate contrarian or denier) or the Seattle Stranger does another hit piece, or someone complains to my Dean, let me explain that global warming will have major impacts on snowpack during future decades and especially after 2050. Increasing CO2 will cause increasing warming during the upcoming century that will reduce snowpack substantially. Some regional climate runs that my group and Professor Eric Salathe completed a few years ago, show major snowpack reductions (see graphic).

But the loss of snowpack during the snowpack has been modest and slow, and certainly not dramatic. And the fact that it has been going on for over a century suggests that part of it is probably natural and not driven by anthropogenically driven increases in greenhouse gases such as CO2. The planet experienced a cool period (the Little Ice Age) from the 1600s to the late 1800s, that produced more snow over our region. With the end of the cool period (probably due to natural causes), snowpack has slowly declined.

Scary headlines and claims of dramatic snowpack loss are counterproductive in many ways. They are clearly not true and thus undermine the credibility of those claiming such losses (activist scientists, politicians, and advocacy organizations). They can result in poor public policy and infrastructure planning. They unnecessarily scare people and make them anxious, an increasing problem (two days ago the Seattle Times had a front page article about a UW Bothell class dealing with dealing with anxiety about climate change).

And then there is the moral/ethical dimension. Scientists and the media must communicate our best understanding of the truth faithfully and not exaggerate/hype to get people to "do the right thing." As I have learned personally, there is a real cost to telling "inconvenient truths", but society can only make wise decisions if it is provided with unvarnished information based on the best science, and including information about uncertainty.

Another issue regards the press releases of universities and other research institutions. There is a tendency to go for dramatic headlines and hype to secure the "currency of the realm" for PR people--lots of clicks and attention. But the contents of the research papers are often distorted in the process. This was clearly an issue for the Oregon State University press release regarding the Mote et al paper, and it occurs all the time here at the University of Washington and at other instituions.

Finally, the spread of such hyped material says something about the current state of online and print media. Apparently, few "reporters" bothered to read the paper they were headlining. Few completed a reality check on the claims. But they were attracted to the big "dramatic" headline and were happy put out the excessive claims as a way of getting attention and "clicks." This is more than an inconvenient truth, but is a challenge for our democracy, since a misinformed public will not make good decisions.

Tuesday, March 13, 2018

Major, long-term, temperature records are being broken this morning, with amazingly high temperatures around Puget Sound. Stunning records.

I knew something was up when I walked my dog this morning around 6:15 AM.... it felt amazingly warm. Then I checked and was floored.

Temperatures at 6 AM were in the mid-60s from Seattle to Tacoma (see graphic, click to expand). 50s over north Seattle. Normal lows this time of the year are around 38-39F.

Sea-Tac Airport was 63F at 6 AM, with an overnight minimum of 62F. The previous minimum temperature temperature record for the date was 50F--but temperatures will fall today....so we will have to see if we beat it.

We are talking about a stunning record low temperature. The plot of temperature at Seattle illustrates this (normal highs and low temps plotted with purple and cyan color lines). The low this morning was roughly 10F higher than our normal high temperatures.

According to my colleagues at the National Weather Service, the 5 AM PDT temperature at Renton, Washington (at 66 F) was the 2nd warmest spot in the lower 48 states....just behind Palm Springs, California (69 F). So what is going on? A very strong easterly wind event with powerful downslope winds. In fact, there is windstorm going on right now in the Cascades foothills, particularly around Enumclaw. And strong downslope winds cause powerful compressional heating of the air, like in your bicycle pump. Thus, the extreme temperatures.

Here are the temperatures and winds around Puget Sound. Look closely (click on image to expand) and you will see that the warmest places have easterly winds.

The pressure gradient across the Cascades is very large now (7.5 hPa across the mountains, higher to the east) and the winds above Sea-Tac Airport (see below) shows a deep layer of easterly flow...and easterly flow that is very strong at low levels. ( the figure shows temperatures in red and wind barbs, time is on the x-axis and height in pressure on the y-axis, 850 is roughly 5000 ft).

The turbulent easterly flow is mixing down to the surface, working against any nighttime cooling by the emission of infrared radiation to space. The strong easterly winds have gusted to 52 mph in Enumclaw and 30-40 mph as far west as the Sound and as far north as Issaquah.

Why are there strong winds and warmth between Seattle and Tacoma? Because of a "weakness" in the Cascades to the east....what meteorologists call "Stampede Gap"---- see the terrain map below.

The winds and high temperatures will fade today....but those of you living between Seattle and Tacoma are experiencing an unusual event today.

Monday, March 12, 2018

As promised, temperatures surged into record territory today (Monday), with many locations getting into the lower to mid-70s. Here are the high temps over the region today. Lower to mid 70s from Bellingham to Medford, Oregon.
With 70s even along the coast (made possible by warm offshore winds). Warmer in Seattle than in the Tri-Cities.

Several locations beat daily records ( Sea-Tac, Olympia, Quillayture, Hoquiam, Bellingham)...and some by a large margin ( Seattle beat the old record by 5F!, Bellingham by 6F)

The 73F today at Sea Tac is the warmest at that location since late last September, as shown by this plot. And 20F above normal.

Getting to 73F at Sea-Tac early in March is quite unusual. It is the earliest day on record that the temperature has risen above 70F at the airport. Now that should impress someone. It impresses me.

The temperature spiked today for a reason: the air was relatively warm due to southerly flow aloft and then we had strong easterly (from the east) flow that supercharged the temperatures by adding strong compressional warming as the air sank down the western slopes of the Cascades and coastal mountains. Plus, few clouds to get into the way.

Here is a plot of wind direction at Sea-Tac Airport for the last 12-h....easterly flow dominated, and some of the winds were quite strong (15-25 mph gusts)

Easterly flow is a relatively unusual direction in central Puget Sound, as shown the the wind rose for March at Sea-Tac airport (see below). To construct this chart one needs a database of March wind speeds and directions at the location. The plot is a polar plot showing direction (with north at the top, south at the bottom, etc.) The frequency of winds from any direction is indicated by the length of the "rose petals." And the relative frequency of various wind speed ranges is shown by the colors. So winds from the south to southwest are the most probable based on the climatology at Sea-Tac. Easterlies are rare.

The easterlies were associated with a strong offshore pressure gradient (higher pressure inland and lower pressure offshore), connected with a very slow-moving front offshore.

The warm temperatures in our area were well timed, considering today was National Nap Day (I kid you not). But rain and cooler temperatures will move into tomorrow.

But something else unusual is about to happen. California is going to be inundated with heavy precipitation, much more that is typical of March. Here is the latest model prediction (NWS GFS model) of accumulated precipitation for the next 10 days. The entire state will get plenty of precipitation, with up to 10 inches in the Sierra Nevada.

What about the vaunted European Center model for the same period? Up to 9 inches of precipitable water in the Sierra, and 2-4 inches over much of the northern part of the State.

Accumulated snowfall... 3-4 feet over a large area of the Sierra Nevada.

The operators of CA reservoirs and dams will have to watch the water levels...they are going to rise substantially during the next week or so. And perhaps this event will quiet the drought talk for CA this summer.

Saturday, March 10, 2018

Temperatures got into the upper 50s today across western Washington...but you haven't seen nothing yet. And unusual easterly winds will really rev up during the next 24 hours or so.

The air aloft has warmed considerably during the past day under southerly flow, and an offshore pressure gradient has initiated easterly (from the east) flow over the Cascades. To illustrate the profound warm up of the next few days, here are the surface air temperature forecasts for 4 PM today (Saturday), 5 PM tomorrow (Sunday), and 4 PM Monday. A big surge of warmth over the next 48h.

Sunday will get into the mid-60s for most. But look at Monday..wow... lots of folks in western WA will experience 70s F.

One driver of the warmth will be easterly flow, with strong downslope warming over the western slopes of the Cascades. Air get compressed as it descends from higher to lower altitudes, and compression causes warming (like you notice when you use an air pump).

Here are the wind gusts predicted for 5 PM on Monday. 20-30 knot gusts from the southeast, particularly from central/south Seattle and eastward. Another area east of Everett. Why? Because there are major low-areas in the Cascades: Stampede Gap south of I90 and the Steven's Pass gap to the north. When there is a large pressure difference across the Cascades, air accelerates eastward through these low regions.

Such southeasterly winds often produce turbulent flights for those landing/taking off from Sea-Tac Airport.

But what will be really unusual next week will be the large amount of precipitation that will hit California due to a strong low center setting up west of northern CA.

Here are the two 72 hr totals for the next next 144 hours. In WA state we will get some sprinkles, but California, and particularly the Sierra Nevada, will get hit by many inches of water and feet of snow over high terrain. And more is predicted the following week. I suspect fewer people will be talking about California drought when the next few weeks are over.

Thursday, March 8, 2018

During the past month, western Washington temperatures have generally been at or below normal, as illustrated by the temperatures at Sea-Tac Airport (see below, purple shows the normal high, blue shows the normal low, actual temps are red). Our high temps have not pushed above normal, but our low temps have dropped below normal on many days.

This cool situation can be traced to the La Nina-like upper air pattern with high pressure offshore and troughing (low pressure) over the Northwest.

But even with this cool weather there have been signs of spring. Days have gotten longer and the sun is higher in the sky. Daffodils and other early bulbs are pushing up and flowering (see below)

And, of course, the surest sign of spring is found in front of many supermarkets:

But this weekend the situation is going to change in a big way. Southerly flow will bring warm air up from the south, and this warmth will be enhanced by low-level offshore (easterly) flow, which will add downslope warming.

There will be no precipitation over the weekend. Western Washington temperatures will rise into the mid-50s on Saturday, mid-60s on Sunday, and upper 60s on Monday, with some hot spots getting even warmer.

Let me show you. Here is the WRF model surface air temperature forecast for 4 PM Saturday. Warm from Everett to Portland and to the southern WA coast, with highs between 56-60F for everyone, and 60-64F east of Seattle and around Portland.

But that is nothing. Here is the forecast for 5 PM Sunday (remember Daylight Savings Time starts that day). 60s everywhere, with upper 60s along the eastern slopes of the Cascades and nearby lowlands, and along the coast. Look closely at the winds--the warmest spots are associated with downslope easterly flow. Sunday is going to be amazing.

But Monday will be even more amazing, with temperatures getting up near 70F for many of you....too bad it will be a work/school day . Here is the surface air temperature forecast for 2 PM Monday. Wow...upper 60s to low-70s from Olympia to Bellingham

Now before we get too excited, let's check on the uncertainty in these forecasts by looking at some ensemble output, which displays the predictions of many forecast runs. Here is the "plume" display of 21 National Weather Service GFS model forecasts of surface air temperature at Seattle. (the model runs started at 10 AM today, Thursday). Progressive warming is found in all the runs, with the highest resolution member (blue) line showing the warmest temperatures (around 67F on Monday). This makes sense because higher resolution results in higher amplitude downslope warming.

So stock up on sunblock and sunglasses, and pull out those short-sleeve shirts....you will need them in two days.

For those of you who enjoy some irony, our warm weather is connected to cool, wet weather hitting central CA next week. Our warm southerly flow is connected with a developing, very large trough that is extending southward in the eastern Pacific (see upper level map for Monday at 5 PM). Strong southwesterly flow will head into California, producing lots of rain at lower elevations and snow for the Sierra crests. Actually good news for them--they need the precipitation.